A conventional known type of position detecting device detects a rotational position of a shaft that is included in an actuator provided for rotationally driving a valve or the like. A position detecting device described in Patent Literature 1 includes a shaft rotatably supported by a casing, a magnetic circuit unit secured to the shaft, and a magnetic detector that is secured to the casing and detects magnetic flux flowing through the magnetic circuit unit. When the shaft rotates, the position of the magnetic detector relative to the position of the magnetic circuit unit changes, and magnetic flux density passing a magnetic detecting surface of the magnetic detector changes. The magnetic detector outputs a voltage signal in accordance with the magnetic flux density passing the magnetic detecting surface. The position detecting device can detect a rotational position of the shaft by using the output of the magnetic detector.